Geoscience Reference
In-Depth Information
Fig. 6.48
Simple analytical petroleum trap filled to the
spill point and leaking through a fault with a lower P
CT
than the caprock (from Ringrose et al.
2000
)
appreciated that faults are highly complex geo-
logical features containing multiple elements act-
ing both as flow conduits and barriers (e.g. Caine
et al.
1996
; Manzocchi et al.
1998
,
2010
). Fault
flow properties also depend on the
in situ
stress
field, a factor we will consider further in the
high-density fractured reservoir section below.
6.7.1.5 Flow Properties
Although sometimes acting as flow barriers,
faults can also be open to cross-flow and can
discriminate between fluids - that is, they have
multiphase flow properties related to capillary
and surface tension effects. These effects can be
subtle and quite substantial. In some cases a fault
can retain an oil column of several 10's of metres
while still being permeable to water.
For the simplest case of a water-wet low-
permeability fault rock, we can define the capil-
lary threshold pressure, P
CT
, required to allow
the non-wetting phase to flow (Manzocchi and
Childs
2013
), e.g. for a static oil-water system:
Fig. 6.47
Small normal fault in an inter-bedded sand-
shale sequence (width of image 2 m)
predict fault seal based on observed across-fault
pressure differences. They defined the SGR for a
specific reservoir interval as:
¼
ʣ
ð
Shale bed thickness
Þ
SGR
100
%
P
CT
¼ ˁ
w
ˁ
o
ð
Þ
gh
o
Fault throw
Other factors used in fault seal analysis
include the clay smear potential (CSP) and the
shale smear factor (SSF), but the SGR method is
most widely applied. Modifications to the SGR
method include corrections for actual clay min-
eral content of shaly beds (Sperrevik et al.
2002
).
The effects of fault seal variation across fault
zones intersecting a multi-layer reservoir interval
can then be mapped using fault juxtaposition
diagrams, to which a measure of seal potential can
be added (Bentley and Barry
1991
; Knipe
1997
).
Despite the utility of proposed predictive tools
for fault seal analysis such as SGR, it should be
where h
o
is the oil column height.
If the fluid pressure of the oil column exceeds
the P
CT
of the fault then oil will flow across the
fault, if not then the fault will be permeable only
to water. Figure
6.48
shows an example of a
simulated leaky fault seal, using these capillary-
controlled flow conditions.
In the case of non-static conditions, where
natural hydrodynamic gradients exist or lateral
pressures are applied (by injection or production)
additional terms for lateral pressure gradients in
the water or oil phase need to be taken into
account (see Manzocchi and Childs
2013
).
